This invention relates to an apparatus for obtaining an information on the motion of a moving element, preferably for determining the speed of a rotating element, for example engine rpm in an automobile.
Arrangements of the kind referred to above are well known. In a conventional arrangement for measuring engine rpm, a sensor detects magnetic irregularities, e.g., teeth or clearances, in a rotating component torsion-resistantly connected with the internal combustion engine. The changing magnetic field induces electrical pulses in the sensor which are transmitted by an electrical transmission line to an evaluation circuit. The evaluation circuit, which is conventional and includes commercially available components, determines the rotational speed of the rotating component from the number of pulses per time unit, or from the time interval between individual pulses, such time interval being inversely proportional to the rotational speed. In one commercially available sensor, a Wiegand sensor, which does not need an electric power supply, as a battery, electrical pulses are produced that have a width independent of the rotational speed of the rotating component.
The measurement of speed, which is derived from the speed-dependent pulses, can be affected by the presence of outside electrical signals or noise which is picked up during transmission of the electrical pulse signals from the moving component to the evaluation circuit. It has been found that it is difficult to suppress, at a reasonable cost, such interfering signals.